This invention relates generally to low-torque, lightweight shafts. More particularly, it concerns an improved shaft, and a laminar structural element and method for its manufacture, the shaft being substantially more durable, yet lighter in weight, than conventional shafts.
Golfers and promoters know that yardage is everything, and that even a few extra yards' distance on a drive may mean the difference in a tournament between a win and a loss. The most significant factor in drive distance for a particular golfer is the leverage obtainable by the golf club's shaft. The lighter the shaft, the more weight that can be added to the head, which on the pro circuit may be traveling in excess of 100 miles per hour (mph) when it strikes the ball. Consequently, the power impacting on the ball by the club's head may be greatly increased by lightening the shaft. (Even when the club is traveling far slower during the golfer's downswing, the club preferably would exhibit proper balance, would flex controllably and would resist torsion, or twisting of the head about the shaft's long axis.) The shaft must flex just so during the backswing and downswing in order to impart the greatest possible angular momentum to the club's head as it strikes the ball. Accuracy is dependent in large part by the torsional resistance of the shaft to twisting during the swing, which can result in pulling or slicing the ball. Flexural performance and torsional resistance in the golf club's shaft require a delicate balance in terms of structural requirements.
Previously, golf club shafts have been made by helically wrapping binder-containing fiber material, e.g. graphite, strips around an armature, e.g. a stainless steel mandrel, to form a slightly tapered, but generally cylindrical, hollow tube and heating the structure to bond the wrapped layers into an integral tubular structure. Polyurethane paints typically are used to coat the bonded structure and the coated structure is polished to produce a finished shaft for assembly into a golf club. It has been suggested that, for resisting the torsional forces incident upon the shaft when the club's head strikes the ball, helical material strips should be biased at an angle transverse to the shaft's long axis, and that preferably alternate strips should be biased at opposite and equal angles of between approximately 20.degree. and 45.degree.. One known patent disclosure suggests adding a singular unbiased, or zero orientation graphite laminate as a middle layer of the shaft's substantially biased-laminar structure. This construction is described and illustrated in U. K. Patent No. 1 446 444, entitled SHAFTS FOR GOLF CLUBS, which was published Aug. 18, 1976, with which familiarity is assumed.
Briefly, the invented golf shaft is fabricated by fabricating a laminar structural element in the form of a planar sheet or blank patterned and sized for spirally, rather than helically, wrapping around a mandrel. The laminar structural element preferably includes oppositely angularly biased plies interposed by a longitudinally or "zero" oriented ply or any suitable pre-impregnated, continuous-fiber material such as boron or carbon-based sheet material such as polyacrylonitrile (PAN). Preferably, another zero orientation ply, prior to wrapping, is placed above one of the bias plies, thereby ensuring complete separation of the opposing bias plies in the finished shaft. Typically, each ply is approximately 0.004-0.006 inch (4-6 mils) thick, producing a laminate, or sandwich, material that is approximately 10-15 mils thick that, when rolled onto a mandrel produces a hollow shaft having an overall wall thickness of approximately 30 mils, rendering the shaft approximately 30% thinner, and significantly lighter in weight, than conventional shafts. Nevertheless, the shaft is because of its unique laminar construction more torque resistant and less subject to shear than those of conventional, helically wrapped, laminar construction. The shaft may be fabricated otherwise by conventional means, whether manual or automated.
The principal object and advantage of the invention is to produce a golf club shaft having a greater stiffness to weight ratio (i.e. effective modulus of elasticity). The invention involves a construction requiring less material to achieve the same stiffness and torque rating as that of much heavier shafts, e.g. 90-100 grams or more. Because there is less material in the construction, the shaft that is of lighter weight, e.g. approximately 50grams, yet exhibits superior performance than, prior constructions.
These and additional object and advantages of the present invention will be more readily understood after a consideration of the drawings and the detailed description of the preferred embodiment.